Interesting question. On the one hand one is limited due to image quality but on the other hand buyers probably search larger resolutions for some special content.
I currently go for 4092 x 2340 pixels. I also have some images with 5376 x 3072 px. This is equal to 2 or 3 x upscale.

I don't think it makes much sense to go further if you sell content which is mostly buyed for mobile usage with smaller image resolutions. I mean even for website banners you would buy smaller sizes.
You need really huge resolutions only for printing.

My camera can take 100 megapixel photos! 

Hasselblad for selling 36 cents via Adobe or 10 cents via SS.
Which lenses do you use? 30, 50 and 90mm with 2.5 to 3.5 apertures?

Sounds like a good business model.